In recent years, nonaqueous electrolyte batteries such as lithium ion secondary batteries have been developed as high-energy density batteries. Nonaqueous electrolyte batteries are expected as power sources for vehicles such as hybrid automobiles and electric automobiles, and as large-size power sources for electric storage. In particular, for the application for vehicles, nonaqueous electrolyte batteries are required to have other characteristics such as rapid charge-discharge performance and long-term reliability. Nonaqueous electrolyte batteries capable of rapid charge and discharge have the advantage of a very short charging time, and can improve vehicle performance in hybrid automobiles and further efficiently recover regenerative energy for use as power.
The rapid charge and discharge are made possible by rapid movements of electrons and lithium ions between positive electrodes and negative electrodes. However, in batteries that use carbon-based negative electrodes including carbonaceous materials, repeated rapid charge and discharge may deposit dendrites of metal lithium on the electrodes. These dendrites can cause internal short-circuits, thereby resulting in heat generation and ignition.
Therefore, batteries have been developed which use, as negative electrode active materials, metal composite oxides in place of the carbonaceous materials. In particular, batteries that use titanium oxides as negative electrode active materials are capable of stable rapid charge and discharge, and also have longer lifetimes as compared with carbon-based negative electrodes.
However, such titanium oxides have higher (nobler) potentials with respect to metal lithium as compared with carbonaceous materials. Moreover, the titanium oxides are low in capacity per mass. For this reason, batteries using these titanium oxides have the problem of being low in energy density.
For example, the electrode potential of titanium oxide is approximately 1.5 V on the basis of metal lithium, and higher (nobler) as compared with the potentials of carbon-based negative electrodes. The potential of the titanium oxide is derived from a redox reaction between Ti3+ and Ti4+ in the electrochemical insertion and extraction of lithium, and thus electrochemically restricted. In addition, rapid lithium ion charging and discharging can be achieved in a stable manner at a high electrode potential on the order of 1.5 V. Therefore, it is substantially difficult to lower the electrode potential in order to improve the energy density.
On the other hand, as for the capacity per unit mass, lithium-titanium composite oxides such as Li4Ti5O12 have a theoretical capacity of around 175 mAh/g. On the other hand, common carbon-based electrode materials have a theoretical capacity of 372 mAh/g. Accordingly, titanium oxides are significantly lower in capacity density as compared with carbon-based negative electrodes. This is because the crystal structures of the titanium oxides have therein small numbers of sites in which lithium can be inserted, or lithium is easily stabilized in the structures, thus decreasing the substantial capacity.
In view of the foregoing, novel materials have been developed. For example, titanium-based oxides that have a monoclinic beta structure and lithium-titanium based oxides have been attracting attention, because of their high theoretical capacity of 335 mAh/g. Furthermore, new electrode materials containing Ti and Nb have been studied, and in particular, monoclinic Nb—Ti based composite oxides represented by TiNb2O7 perform charge compensation of Ti from tetravalence to trivalence and Nb from pentavalence to trivalence during lithium insertion, thus leading to a high theoretical capacity of 387 mAh/g.
However, these negative electrode active materials cause strong reactions at potentials that are lower than 1.3 V (vs. Li+/Li). In particular, in batteries including nonaqueous electrolytic solutions including fluorine-containing supporting salts such as LiPF6 or LiBF4, decomposition reactions of the nonaqueous electrolytic solutions proceed with extraction of lithium ions included in the active materials, thereby generating inorganic films such as LiF and Li2CO3. These inorganic films interfere with the insertion and extraction of lithium ions to and from the active material, and thus act as resistance. As a result, there is the problem of degraded cycle characteristics.